Re: Stop scheduler on panic

On Thursday, November 17, 2011 11:58:03 am Andriy Gapon wrote:
> on 17/11/2011 18:37 John Baldwin said the following:
> > On Thursday, November 17, 2011 4:47:42 am Andriy Gapon wrote:
> >> on 17/11/2011 10:34 Andriy Gapon said the following:
> >>> on 17/11/2011 10:15 Kostik Belousov said the following:
> >>>> I have the following change for eons on my test boxes. Without it,
> >>>> I simply cannot get _any_ dump.
> >>>>
> >>>> diff --git a/sys/cam/cam_xpt.c b/sys/cam/cam_xpt.c
> >>>> index 10b89c7..a38e42f 100644
> >>>> --- a/sys/cam/cam_xpt.c
> >>>> +++ b/sys/cam/cam_xpt.c
> >>>> _at__at_ -4230,7 +4230,7 _at__at_ xpt_done(union ccb *done_ccb)
> >>>>  			TAILQ_INSERT_TAIL(&cam_simq, sim, links);
> >>>>  			mtx_unlock(&cam_simq_lock);
> >>>>  			sim->flags |= CAM_SIM_ON_DONEQ;
> >>>> -			if (first)
> >>>> +			if (first && panicstr == NULL)
> >>>>  				swi_sched(cambio_ih, 0);
> >>>>  		}
> >>>>  	}
> >>>
> >>> I think that this (or similar) change should go into the patch and the tree.
> >>>
> >>
> >> And, BTW, I still would like to do something like the following (perhaps with
> >> td_oncpu = NOCPU and td_flags &= ~TDF_NEEDRESCHED also moved to the common code):
> >>
> >> Index: sys/kern/sched_ule.c
> >> ===================================================================
> >> --- sys/kern/sched_ule.c	(revision 227608)
> >> +++ sys/kern/sched_ule.c	(working copy)
> >> _at__at_ -1790,7 +1790,6 _at__at_ sched_switch(struct thread *td, struct thread *new
> >>  	td->td_oncpu = NOCPU;
> >>  	if (!(flags & SW_PREEMPT))
> >>  		td->td_flags &= ~TDF_NEEDRESCHED;
> >> -	td->td_owepreempt = 0;
> >>  	tdq->tdq_switchcnt++;
> >>  	/*
> >>  	 * The lock pointer in an idle thread should never change.  Reset it
> >> Index: sys/kern/kern_synch.c
> >> ===================================================================
> >> --- sys/kern/kern_synch.c	(revision 227608)
> >> +++ sys/kern/kern_synch.c	(working copy)
> >> _at__at_ -406,6 +406,8 _at__at_ mi_switch(int flags, struct thread *newtd)
> >>  	    ("mi_switch: switch must be voluntary or involuntary"));
> >>  	KASSERT(newtd != curthread, ("mi_switch: preempting back to ourself"));
> >>
> >> +	td->td_owepreempt = 0;
> >> +
> >>  	/*
> >>  	 * Don't perform context switches from the debugger.
> >>  	 */
> >> Index: sys/kern/sched_4bsd.c
> >> ===================================================================
> >> --- sys/kern/sched_4bsd.c	(revision 227608)
> >> +++ sys/kern/sched_4bsd.c	(working copy)
> >> _at__at_ -940,7 +940,6 _at__at_ sched_switch(struct thread *td, struct thread *new
> >>  	td->td_lastcpu = td->td_oncpu;
> >>  	if (!(flags & SW_PREEMPT))
> >>  		td->td_flags &= ~TDF_NEEDRESCHED;
> >> -	td->td_owepreempt = 0;
> >>  	td->td_oncpu = NOCPU;
> >>
> >>  	/*
> >>
> >> Does anybody see any potential problems with such a change?
> > 
> > Hmm, does this mean the preemption will be lost if you break into the
> > debugger and continue in the non-panic case?
> 
> Not sure which exact scenario you have in mind.
> Please note that the above diff just moves resetting of td_owepreempt to an
> earlier place.  As far as I can see there are no checks of td_owepreempt value
> between the new place and the old places.

I'm worried that you are clearing td_owepreempt even in cases where a context
switch is not performed.  So say you enter DDB with td_owepreempt set and that
DDB bails on a context switch.  With your change it will now clear td_owepreempt
and "lose" the preemption.

-- 
John Baldwin